Method for determining a wrong synchronization of a receiver with a satellite, associated receiver and computer program product

ABSTRACT

A method for determining a wrong synchronization of a receiver with a satellite, associated receiver and computer program product, is implemented after the acquisition phase and includes the following steps:
         generating first and second test signals;   for each correlation interval, determining a first prompt correlator corresponding to the correlation value between the received signal and the first test signal, and a second prompt correlator corresponding to the correlation value between the received signal and the second test signal;   determining first and second energy values corresponding to the energy of the first and second correlators, respectively;   determining a wrong synchronization indicator based on the difference between the first and second energy values; and   detecting a wrong synchronization based on this indicator.

The present invention relates to a method for determining a wrong synchronization of a receiver with a satellite during a phase for acquiring a navigation signal coming from this satellite.

Such a satellite is part of a global navigation satellite system (GNSS).

In general, a GNSS system is made up of a plurality of satellites allowing a mobile receiver to determine its position in a land-based plane of reference, its speed and the time.

There are currently several GNSS systems, which in particular include the GPS system, the GLONASS system and the GALILEO system, which is expected to be brought online soon.

The satellites of such a GNSS system are able to emit electromagnetic signals in particular comprising navigation information.

Each item of navigation information generally comprises data relative to the transmission time by the satellite of the corresponding signal and the current position of the satellite. In particular, the data relative to the current position of the satellite generally contain the almanac providing a rough position of the satellite and the ephemerids giving the exact current position of the satellite.

The item of navigation information is carried by a carrier wave and modulated by a spreading code specific to each satellite. Thus, the signals are transmitted by the satellites using a spread spectrum technique.

The receiver is able to receive the signals emitted by the satellites and to extract the navigation information therefrom in particular to determine the distance to the satellite that transmitted the corresponding signal. This distance, also called pseudo-distance, is determined by analyzing the propagation delay of the corresponding signal.

To determine its position, its speed and the time, the receiver performs a digital processing of the navigation information from at least three different satellites.

In practice, to have a more precise position, the receiver needs navigation information from at least four different satellites.

More specifically, to acquire the navigation information from a given satellite, the receiver implements two phases processing the signals from this satellite.

During an initial phase, called acquisition phase in the state of the art, the receiver generates a local signal in particular containing a local spreading code showing the image of the spreading code of the satellite.

Since the receiver does not initially know its position, the local signal is not synchronized with the received signal. This in particular means that the local signal is carrier frequency-shifted from the received signal by a value called Doppler value, and the spreading code of the received signal is delayed from the local spreading code by a value called delay value.

The receiver then searches for a peak in the correlations between the local signal and the received signal by trying different Doppler and delay values.

When a peak is detected, the receiver determines the Doppler and delay values corresponding to this peak and, from these values, launches a following phase, called tracking phase in the state of the art.

During the tracking phase, the receiver regularly updates the Doppler and delay values, and extracts the item of navigation information from the signal transmitted by the satellite in particular using the local spreading code and the determined Doppler and delay values.

At the end of the acquisition phase, the receiver is considered to have synchronized itself with the satellite, or has “locked on” to the satellite.

This in particular means that the receiver was able to find the Doppler and delay values relative to this satellite to initiate the tracking phase.

Sometimes, the receiver synchronizes its local signal corresponding to the desired satellite with the signal received from another satellite, which leads to an erroneous distance measurement, and therefore potentially a wrong position.

In this case, this is a wrong synchronization, or a false “lock”.

This for example occurs when the correlation between the local signal and the signal received from the sought satellite provides less energy than the correlation with the signal received from another satellite, due to a large received power deviation.

Different methods exist in the state of the art making it possible to avoid such a wrong synchronization.

Thus, one method, used conventionally, consists of verifying the consistency between the position of the satellite computed from ephemerids contained in the item of navigation information and that computed from the almanac, containing the identifiers of the satellites, unlike the ephemerids. An inconsistency between these values therefore indicates a wrong synchronization.

However, this method is not completely satisfactory. In particular, it requires collecting all of the ephemerids contained in the navigation information, which is relatively time-consuming. In practice, this may take up to two minutes.

One can then see that this is detrimental to the operation of the receiver, in particular after the satellite is hidden, and does not make it possible to ensure continuous service.

The present invention aims to resolve this drawback.

To that end, the invention relates to a method for detecting a wrong synchronization of a receiver with a satellite during a phase for acquiring a navigation signal from this satellite, the satellite being part of a global navigation satellite system, the navigation signal being of the simple or complex type; the signal of each type comprising a data channel including an item of navigation information modulated by a data spreading code specific to this satellite and carried by a carrier wave. The complex-type signal further comprising a pilot channel including a pilot spreading code specific to this satellite and carried by a carrier wave; during the acquisition phase, the receiver being able to receive the navigation signal transmitted by the satellite, to generate a local image of the data spreading code and a local image of the carrier wave for a simple or complex signal, and further to generate a local image of the pilot spreading code for a complex signal.

The method is implemented after the acquisition phase during a convergence phase and comprising the following steps:

-   -   generating a first test signal comprising the local image of the         carrier wave and a first test spreading code, and a second test         signal comprising the local image of the carrier wave and a         second test spreading code; for a simple signal, the first and         second test spreading codes being codes whose sum corresponds to         the local image of the data spreading code; for a complex         signal, the first and second test spreading codes corresponding         to the local images of the data spreading code and the pilot         spreading code, respectively;     -   for each correlation interval of a plurality of correlation         intervals, defining a first prompt correlator corresponding to         the correlation value between the received signal and the first         test signal, and a second prompt correlator corresponding to the         correlation value between the received signal and the second         test signal;     -   determining first and second energy values corresponding to the         energy of the first and second correlators, respectively, over         all of the correlation intervals;     -   determining a wrong synchronization indicator based on the         difference between the first and second energy values;     -   detecting a wrong synchronization when the wrong synchronization         indicator is above a predetermined threshold.

According to other advantageous aspects of the invention, the detection method comprises one or more of the following features, considered alone or according to all technically possible combinations:

-   -   the wrong synchronization indicator is determined using the         following expression:

${{Ind} = \frac{{E_{2} - E_{1}}}{E_{2} + E_{1}}},$

where

E₁ is the first energy value; and

E₂ is the second energy value;

-   -   the first energy value corresponds to the sum of the squares of         norms of the first correlators over all of the correlation         intervals, and the second energy value corresponds to the sum of         the squares of norms of the second correlators over all of the         correlation intervals;     -   the first and second test spreading codes are decorrelated;     -   the local image of the data spreading code and the first and         second test spreading codes each define a code period formed by         a plurality of chips, each chip assuming a value of a         discretized state from among a set of discrete states, one of         these states, called zero state, corresponding to the zero value         of this code;     -   for a simple-type signal, the period of the first test spreading         code is made up of even chips of the local image of the data         spreading code, each pair of said chips being spaced apart by a         chip whose discretized state corresponds to the zero state; and         the period of the second test spreading code is formed by odd         chips of the local image of the data spreading code, each pair         of said chips being spaced apart by a chip whose discretized         state corresponds to the zero state; and     -   for a simple-type signal, the first half of the period of the         first test spreading code is made up of corresponding chips of         the local image of the data spreading code, and the second half         is made up of chips whose discretized state corresponds to the         zero state and the first half of the period of the second test         spreading code is made up of chips whose discretized state         corresponds to the zero state, and the second half is made up of         chips corresponding to the local image of the data spreading         code.

The invention also relates to a computer program product including software instructions which, when implemented by computer equipment, carry out a method as defined above.

The invention also relates to a receiver carrying out the method as previously defined.

These features and advantages of the invention will appear upon reading the following description, provided solely as a non-limiting example, and done in reference to the appended drawings, in which:

FIG. 1 is a schematic view of a global navigation satellite system and a receiver according to the invention;

FIG. 2 is a schematic view of an autocorrelation function used during the operation of the system of FIG. 1;

FIG. 3 is a schematic view of an inter-correlation function used during the operation of the system of FIG. 1;

FIG. 4 is a detailed view of the receiver of FIG. 1;

FIG. 5 is a flowchart of a detection method carried out by the receiver of FIG. 4;

FIGS. 6 and 7 are schematic views illustrating spreading codes used during the implementation of the method of FIG. 5 for a simple signal; and

FIGS. 8 and 9 are schematic views illustrating spreading codes used during the implementation of the method of FIG. 5 for a complex signal.

In the rest of the description, the expression “substantially equal to” refers to an equality relationship to within plus or minus 10%.

FIG. 1 shows a global navigation satellite system (GNSS) 10.

In reference to this FIG. 1, the positioning system 10 includes a plurality of satellites Sat_(n) positioned in different orbits around the Earth for which the positioning system 10 is established.

The total number of satellites Sat_(n) is for example equal to 30.

The index n corresponds to an identifier of each satellite Sat_(n), and for example varies between 1 and 30.

Each satellite Sat_(n) is able to transmit electromagnetic signals S toward part of the Earth's surface 14 that it is flying over.

In particular, the satellites Sat_(n) are arranged such that at least four satellites Sat_(n) are able to transmit electromagnetic navigation signals S toward substantially each point of the Earth's surface 14.

The current position of each satellite Sat_(n) is characterized by the ephemerids relative to this satellite or by the almanac thereof.

As is known in itself, the ephemerids make it possible to determine the exact position of the satellite Sat_(n), while the almanac provides a rough position.

Each signal S transmitted by each of the satellites Sat_(n) comprises a data channel including an item of navigation information.

In particular, such a signal S comprises navigation information modulated by a data spreading code C_(n) ^(d)(φ_(c)) specific to the satellite Sat_(n) having transmitted this signal.

The modulated navigation information is carried by a carrier wave exp(−jφ_(p)) using a technique known in itself.

A signal S comprising only one data channel is hereinafter referred to as a “simple signal”.

The signal S transmitted by at least some of the satellites Sat_(n) further comprises a pilot channel including a pilot spreading code C_(n) ^(p)(φ_(c)) specific to the satellite Sat_(n) having transmitted this signal.

Such a signal S further comprising a pilot channel is hereinafter referred to as a “complex signal”.

Each item of navigation information in particular comprises the transmission time of the corresponding signal, the ephemerids and the almanac of the satellite Sat_(n) at the time of transmission of the signal S.

Each data C_(n) ^(d)(φ_(c)) or pilot C_(n) ^(p)(φ_(c)) spreading code has a binary code of the pseudorandom noise (PRN) type.

Each data C_(n) ^(d)(φ_(c)) or pilot C_(n) ^(p)(φ_(c)) spreading code is a periodic code with a code period denoted L_(c) and expressed as a whole number of reference units.

The reference unit is for example a chip whose duration is denoted T_(chip) and expressed in seconds.

A “chip” refers to a reference unit corresponding to a slot of a pseudorandom noise-type code.

Each chip assumes a value of a discretized state from among a set of discretized states, one of these states, called zero state, corresponding to the zero value of this code.

The data C_(n) ^(d)(φ_(c)) or pilot C_(n) ^(p)(φ_(c)) spreading codes are characterized by their autocorrelation functions R_(n,n) respectively determined using the following formulas:

${{R_{n,n}(\tau)} = {\frac{1}{L_{c}}{\int_{0}^{L_{c}}{{C_{n}^{d}(u)}{C_{n}^{d}\left( {u + \tau} \right)}{du}}}}},{{R_{n,n}(\tau)} = {\frac{1}{L_{c}}{\int_{0}^{L_{c}}{{C_{n}^{p}(u)}{C_{n}^{p}\left( {u + \tau} \right)}{{du}.}}}}}$

In particular, at point τ=0, called reference point, the autocorrelation function R_(n,n) assumes its maximum value V_(max). For τ≦−T_(chip) and τ≧T_(chip), the autocorrelation function R_(n,n) assumes very small values V in view of V_(max), i.e., V<<V_(max).

One example of such an autocorrelation function R_(n,n) of a data spreading code C_(n) ^(d)(φ_(c)) is illustrated schematically in FIG. 2.

The data C_(n) ^(d)(φ_(c)) or pilot C_(n) ^(p)(φ_(c)) spreading codes corresponding to the different satellites Sat_(n) are quasi-perpendicular. In other words, the inter-correlation function R_(n,j) between each pair of different spreading codes C_(n) ^(d)(φ_(c)) and C_(i) ^(d)(φ_(c)) or C_(n) ^(p)(φ_(c)) and C_(i) ^(p)(φ_(c)) assumes negligible values V in view of V_(max), the inter-correlation functions R_(n,j) for these codes respectively being determined by the following formulas:

${{R_{n,j}(\tau)} = {\frac{1}{L_{c}}{\int_{0}^{L_{c}}{{C_{n}^{d}(u)}{C_{j}^{d}\left( {u + \tau} \right)}{du}}}}},{{R_{n,j}(\tau)} = {\frac{1}{L_{c}}{\int_{0}^{L_{c}}{{C_{n}^{p}(u)}{C_{j}^{p}\left( {u + \tau} \right)}{{du}.}}}}}$

One example of such an intercorrelation function R_(n,j) for different data spreading codes C_(n) ^(d)(φ_(c)) is illustrated schematically in FIG. 3.

Each data spreading code C_(n) ^(d)(φ_(c)) makes it possible to modulate the navigation information transmitted by the satellite Sat_(n).

When a satellite Sat_(n) only transmits simple signals, these signals are demodulated using a local image C_(n) ^(d)(φ_(cloc)) of the data spreading code C_(n) ^(d)(φ_(c)) corresponding to this satellite.

When a satellite Sat_(n) transmits complex signals, each of the two signals is demodulated using a spreading code that is the local image C_(n) ^(d)(φ_(cloc)) of the data spreading code C_(n) ^(d)(φ_(c)) or the local image C_(n) ^(p)(φ_(cloc)) of the pilot spreading code C_(n) ^(p)(φ_(c)) corresponding to this satellite.

These signals are demodulated using different techniques for transmitting complex signals known in themselves.

Thus, for example, under the “TDMA (Time Division Multiple Access) Multiplexing” technique, the odd chips of the composite spreading code correspond to the corresponding data spreading code C_(n) ^(d)(φ_(c)), and the even chips of the composite spreading code correspond to the corresponding pilot spreading code C_(n) ^(p)(φ_(c)).

According to the “CDMA (Code Division Multiple Access) Multiplexing” technique, each complex signal is sent via two carrier waves at the same frequency and synchronously. In this case, the composite spreading code corresponds to the sum of the corresponding data C_(n) ^(d)(φ_(c)) and pilot C_(n) ^(p)(φ_(c)) spreading codes.

According to the example embodiment described below, the positioning system 10 is the GPS system (Global Positioning System).

Of course, other example embodiments are also possible.

The signals S transmitted by at least some of the satellites Sat_(n) are received by a receiver 20.

The receiver 20 is for example a portable electronic device.

The receiver 20 is for example able to move on or near the Earth's surface 14 with a variable speed.

The receiver 20 is able to receive signals S from satellites Sat_(n), and to extract the navigation information from these signals S to deduce its current position, its current speed and the time, as will be explained later.

The receiver 20 is illustrated in more detail in FIG. 4.

Thus, in reference to this FIG. 4, the receiver 20 includes an antenna 22, an acquisition module 24, a tracking module 26, a detection module 28 according to the invention, a control module 30 and hardware resources.

The modules 24, 26, 28 and 30 for example assume the form of software programs that are implemented by the hardware resources provided to that end, such as a processor, random-access memory, read-only memory, etc. The hardware resources are for example battery-powered.

In particular, the read-only memory of the receiver 20 is able to store images of the data C_(n) ^(d)(φ_(c)), and optionally pilot C_(n) ^(p)(φ_(c)), spreading codes of each satellite Sat_(n).

The antenna 22 is able to receive electromagnetic signals S_(r) corresponding to the signals S transmitted by the satellites Sat_(n) when the latter are in range of its visibility.

The control module 30 is able to control the operation of the modules 24, 26 and 28.

The acquisition module 24 is able to carry out an acquisition phase for the signals S_(r) using techniques known in themselves.

The tracking module 26 is able to carry out a tracking phase for the signals S_(r) using techniques known in themselves.

Lastly, the detection module 28 is able to carry out a convergence phase, which is transitional between the acquisition phase and the tracking phase. The convergence phase in particular comprises a method 100 for detecting a wrong synchronization according to the invention. This method will be described in detail below.

The operation of the receiver 20 will now be explained.

Each time the receiver 20 is started, the control module 30 initiates a plurality of acquisition channels for all of the satellites Sat_(n). Each of these channels makes it possible to acquire the item of navigation information from the satellite Sat_(n) with which it is associated, when this satellite Sat_(n) is in the visibility range of the antenna 22.

The operation of the receiver 20 on each acquisition channel is substantially similar. Thus, only the operation of the receiver 20 on one channel will be explained below.

This channel is for example associated with the satellite Sat_(n), hereinafter called sought satellite. It is further assumed that the satellite Sat_(n) is situated in the visibility range of the antenna 22 and that this satellite is able to transmit simple and complex signals.

For a simple signal, the receiver 20 generates a local data signal S_(loc) ^(d) including a local carrier wave exp(−jφ_(ploc)) and a local data spreading code C_(n) ^(d)(φ_(cloc)) corresponding to a local image of the data spreading code C_(n) ^(d)(φ_(c)) of the sought satellite.

The local data signal S_(loc) ^(d) as a function of time t is then written in the following form:

S _(loc) ^(d)(t)=exp(−jφ _(ploc)(t)·C _(n) ^(d)(φ_(cloc)(t)),

with j²=−1.

For a complex signal, the receiver 20 further generates a local pilot signal S_(loc) ^(p) including a local carrier wave exp(−jφ_(ploc)) and a local pilot spreading code C_(n) ^(p)(φ_(cloc)) corresponding to a local image of the pilot spreading code C_(n) ^(p)(φ_(c)) of the sought satellite.

Then, the control module 30 launches the execution of the acquisition phase, which is then carried out by the acquisition module 24.

In particular, during the acquisition phase, the acquisition module 24 determines a Doppler value and a delay value of the received signal S_(r) relative to the local data S_(loc) ^(d) or pilot S_(loc) ^(p) signal.

The Doppler value corresponds to the frequency shift of the local carrier wave exp(−jφ_(p1oc)) relative to the carrier wave exp(−jφ_(p)) of the received signal S_(r).

In the described example, for a simple signal, the delay value corresponds to the delay of the data spreading code C_(n) ^(d)(φ_(c)) of this received signal relative to the local data spreading code C_(n) ^(d)(φ_(cloc)).

For a complex signal, the delay value corresponds to the delay of the pilot spreading code C_(n) ^(p)(φ_(c)) of this received signal relative to the local pilot spreading code C_(n) ^(p)(φ_(cloc)).

In both cases, the delay values are determined using known techniques that in particular comprise computing three types of correlations.

A first type of correlation, called prompt correlation, consists of computing correlations between the received signal S_(r) and the local data S_(loc) ^(d) or pilot S_(loc) ^(p) signal.

A second type of correlation, called advance correlation, consists of computing correlations between the received signal S_(r) and a signal corresponding to the local data S_(loc) ^(d) or pilot S_(loc) ^(p) signal in which the local spreading code C_(n) ^(d)(φ_(cloc)+d) or C_(n) ^(p)(φ_(cloc)+d) is shifted forward by a value d comprised between 0 and T_(chip).

A third type of correlation, called delay correlation, consists of computing correlations between the received signal S_(r) and a signal corresponding to the local data S_(loc) ^(d) or pilot S_(loc) ^(p) signal in which the local spreading code C_(n) ^(d)(φ_(cloc)−d) or C_(n) ^(p)(φ_(cloc)−d) is delay-shifted by the same value d.

At the end of the acquisition phase, the receiver 20 synchronizes the local data S_(loc) ^(d) or pilot S_(loc) ^(p) signal with the signal S transmitted by the sought satellite Sat_(n) by using the determined Doppler and delay values.

Then, the control module 30 launches the execution of the convergence phase, and in particular of the method 100 for detecting a wrong synchronization. This method is in particular carried out by the detection module 28.

The convergence phase enslaves the delay value of the local spreading code C_(n) ^(d)(φ_(cloc)) or C_(n) ^(p)(φ_(cloc)) and the Doppler value of the local carrier wave exp(−jφ_(ploc)), on the received signal S_(r), using code and carrier tracking loops, in particular owing to the three aforementioned types of correlation.

This transitional phase makes it possible to cause the local spreading code C_(n) ^(d)(φ_(cloc)) or C_(n) ^(p)(φ_(cloc)) and the local carrier wave exp(−jφ_(ploc)) to coincide precisely with the spreading code C_(n) ^(d)(φ_(c)) or C_(n) ^(p)(φ_(c)) and the carrier wave exp(−jφ_(p)) of the received satellite signal S_(r).

Furthermore, the detection method 100 carried out during this phase makes it possible to detect a wrong synchronization of the corresponding acquisition channel with a satellite Sat_(p) other than the sought satellite Sat_(n).

In particular, the detection method 100 optionally makes it possible to detect a wrong synchronization with a non-zero Doppler value by determining the inconsistency between the frequency of the local spreading code C_(n) ^(d)(φ_(cloc)) or C_(n) ^(p)(φ_(cloc)) and the frequency of the local carrier exp(−jφ_(ploc)).

If such a wrong synchronization is not detected, the detection method 100 makes it possible to detect a wrong synchronization with a zero Doppler value, as will be explained below.

In particular, when a wrong synchronization occurs, the Doppler and delay values determined by the acquisition module 24 correspond to the signal S transmitted by a satellite Sat_(p) other than the sought satellite Sat_(n).

One can then see that in this case, the signals S_(loc) ^(d) or S_(loc) ^(p) and S cannot be synchronized correctly. This is therefore a wrong synchronization and a false lock.

When the detection module 28 detects a wrong synchronization, the control module 30 launches the acquisition phase again.

When the detection module 28 does not detect a wrong synchronization, the control module 30 launches the tracking phase, which is then carried out by the tracking module 26.

In particular, during the tracking phase, the tracking module 26 regularly updates the Doppler and delay values, which allows it to demodulate the received signal S_(r) and extract the corresponding item of navigation information therefrom. It subsequently sends this information to the control module 30.

Lastly, the control module 30 consolidates all of the information acquired by the set of acquisition channels and deduces the position of the receiver 20, its speed and the time therefrom.

The method for detecting a wrong synchronization will now be explained in detail in reference to FIG. 5, showing a flowchart of its steps.

During the initial step 110, the detection module 28 generates a first test signal S_(T) ¹ and a second test signal D_(T) ².

The first test signal S_(T) ¹ comprises the local carrier wave exp(−jφ_(ploc)) and a first test spreading code C_(n) ¹(φ_(cloc)), and is determined according to the following relationship:

S _(T) ¹(t)=exp(−jφ _(ploc)(t))·C _(n) ¹(φ_(cloc)(t)).

The second test signal S_(T) ² comprises the local carrier wave exp(−jφ_(ploc)) and a second test spreading code C₂ ²(φ_(cloc)), and is determined according to the following relationship:

S _(T) ²(t)=exp(−jφ _(ploc)(t))·C _(n) ²(φ_(cloc)(t)).

When the received signal S_(r) is of the simple type, the first and second test spreading codes are decorrelated codes whose sum corresponds to the local data spreading code C_(n) ^(d)(φ_(cloc)), i.e.:

C _(n) ¹(φ_(cloc))+C _(n) ²(φ_(cloc))=C _(n) ^(d)(φ_(cloc)).

According to one alternative embodiment, the period L_(c) of the first test spreading code C_(n) ¹(φ_(cloc)) is made up of even chips of the local data spreading code C_(n) ^(d)(φ_(cloc)), each pair of said chips being spaced apart by a chip whose discretized state corresponds to the zero state.

According to the same alternative embodiment, the period L_(c) of the second test spreading code C_(n) ²(φ_(cloc)) is made up of odd chips of the local data spreading code C_(n) ^(d)(φ_(cloc)), each pair of said chips being spaced apart by a chip whose discretized state corresponds to the zero state.

One example of these three spreading codes C_(n) ¹(φ_(cloc)), C_(n) ^(d)(φ_(cloc)) is illustrated in FIG. 6.

According to another alternative embodiment, the first half of the period L_(c) of the first test spreading code C_(n) ¹(φ_(cloc)) is made up of corresponding chips of the local data spreading code C_(n) ^(d)(φ_(cloc)), and the second half is made up of chips whose discretized state corresponds to the zero state.

According to the same alternative embodiment, the first half of the period L_(c) of the second test spreading code C_(n) ²(φ_(cloc)) is made up of chips whose discretized state corresponds to the zero state, and the second half is made up of corresponding chips of the local data spreading code C_(n) ^(d)(φ_(cloc)).

An example of the three spreading codes C_(n) ¹(φ_(cloc)), C_(n) ²(φ_(cloc)), C_(n) ^(d)(φ_(cloc)) according to this alternative embodiment is illustrated in FIG. 7.

When the received signal S_(r) is of the complex type, the first and second test spreading codes C_(n) ¹(φ_(cloc)), C_(n) ²(φ_(cloc)) correspond to the local data C_(n) ^(d)(φ_(cloc)) and pilot C_(n) ^(p)(φ_(cloc)) spreading codes, respectively.

FIG. 8 illustrates an example of the test spreading codes C_(n) ¹(φ_(cloc)), C_(n) ²(φ_(cloc)) and a composite spreading code C_(n)(φ_(cloc)) corresponding to the sum of these codes for a complex signal sent using the TDMA technique.

FIG. 9 illustrates an example of the test spreading codes C_(n) ¹(φ_(cloc)), C_(n) ²(φ_(cloc)) and a composite spreading code C_(n)(φ_(cloc)) corresponding to the sum of these codes for a complex signal sent using the CDMA technique.

The following steps 115 and 120 are carried out in parallel, during the convergence phase. The steps 135, 140, 150 and 160 are carried out consecutively at the end of the convergence phase.

During step 115, the detection module 28 determines a plurality of correlation intervals.

Each correlation interval is subsequently identified by the index k varying between 1 and K_(max), K_(max) being the total number of correlation intervals over the duration of the convergence phase.

The correlation intervals for example have substantially the same duration T, which is for example equal to 20 ms.

During step 120, for each correlation interval k, the detection module 28 determines a first prompt correlator Z_(p) ¹(k) and a second prompt correlator Z_(p) ²(k).

In particular, the first prompt correlator Z_(p) ¹(k) corresponds to the correlation value between the received signal S_(r) and the first test signal S_(T) ¹ in the correlation interval k; this correlation value is determined by the following formula:

${Z_{p}^{1}(k)} = {\frac{1}{T}{\int_{kT}^{{({k + 1})}T}{{S_{r}(t)}{S_{T}^{1}(t)}{{dt}.}}}}$

The second prompt correlator Z_(p) ²(k) corresponds to the correlation value between the received signal S_(r) and the second test signal S_(T) ² in the correlation interval k; this correlation value is determined by the following formula:

${Z_{p}^{2}(k)} = {\frac{1}{T}{\int_{kT}^{{({k + 1})}T}{{S_{r}(t)}{S_{T}^{2}(t)}{{dt}.}}}}$

During step 135, carried out at the end of the convergence phase, the detection module 28 determines whether it involves a wrong synchronization with a non-zero Doppler value.

To that end, the detection module 28 determines an inconsistency value V_(inc) between the frequency of the local data C_(n) ^(d)(φ_(cloc)) or pilot C_(n) ^(p)(φ_(cloc)) spreading code and the frequency of the local carrier exp(−jφ_(ploc)).

The inconsistency value V_(inc) is determined using the following expression:

${V_{inc} = {{c\left\lbrack {\frac{F_{cloc}}{F_{cnom}} - \frac{F_{ploc}}{F_{pnom}}} \right\rbrack}}},$

where

F_(cloc)=(φ_(cloc)(t+Δt)−φ_(cloc)(t))/Δt is the frequency of the local spreading code;

F_(ploc)=(φ_(ploc)(t+Δt)−φ_(ploc)(t))/2π/Δt is the frequency of the local carrier;

F_(cnom) and F_(pnom) are predetermined nominal values of the frequency of the local data C_(n) ^(d)(φ_(cloc)) or pilot C_(n) ^(p)(φ_(cloc)) spreading code and the frequency of the local carrier exp(−jφ_(ploc)), respectively; and

c is the speed of light.

In the described example embodiment, corresponding to the GPS L1 C/A signals, the nominal values F_(cnom) and F_(pnom) are chosen as follows:

-   -   F_(cnom)=1.023 MHz, F_(pnom)=1575.42 MHz.

Then, the detection module 28 compares the inconsistency value V_(inc) with a predetermined inconsistency threshold.

When the inconsistency value is above the inconsistency threshold, the detection module 28 determines whether it involves a wrong synchronization with a non-zero Doppler value. In this case, the control module 30 launches the acquisition phase again.

Otherwise, the detection module 28 determines whether it involves a wrong synchronization with a non-zero Doppler value, through the following steps.

In particular, during step 140, the detection module 28 determines a wrong synchronization indicator Ind based on the determined correlators.

During the following step 140, the detection module 28 determines first and second energy values E₁ and E₂ corresponding to the energy of the first and second correlators Z_(p) ¹(k) and Z_(p) ²(k), respectively, over all of the correlation intervals.

The energy values E₁ and E₂ are determined using the following formulas:

${E_{1} = {\sum\limits_{{k = 1},\ldots \mspace{11mu},K_{\max}}{{Z_{p}^{1}(k)}}^{2}}},{E_{1} = {\sum\limits_{{k = 1},\ldots \mspace{11mu},K_{\max}}{{Z_{p}^{2}(k)}}^{2}}},$

where the notation ∥*∥ indicates a norm.

During the following step 150, the detection module 28 determines a wrong synchronization indicator Ind based on the difference between the first and second energy values E₁ and E₂, using the following formula:

${Ind} = {\frac{{E_{2} - E_{1}}}{E_{2} + E_{1}}.}$

During the following step 160, the detection module compares the indicator Ind with a predetermined threshold ε and detects a wrong synchronization when this indicator Ind is above the threshold ε. In this case, the control module 30 launches the acquisition phase again.

Otherwise, the detection module 28 determines that it does not involve a wrong synchronization and the control module 30 launches the tracking phase.

One can then see that the present invention has a certain number of advantages.

Thus, the method according to the invention makes it possible to detect a wrong synchronization very quickly, just after the acquisition phase.

This then makes it possible to avoid the need to acquire ephemerids contained in the item of navigation information and to thus decrease the detection time for a wrong synchronization.

The use of the method is particularly advantageous when satellites are concealed, since the method makes it possible to ensure continuous service.

The particular advantage of the invention lies in the fact that in order to detect a wrong synchronization, it is not necessary to know the specific form of the spreading codes used by the satellites. Indeed, the detection method can be applied to any type of spreading code without any particular adaptation being necessary.

Of course, many other example embodiments of the invention are also possible. 

What is claimed is:
 1. A method for detecting a wrong synchronization of a receiver with a satellite during a phase of acquiring a navigation signal from said satellite, said satellite being part of a global navigation satellite system, said navigation signal, of the so-called simple type, having only one data channel including an item of navigation information modulated by a data spreading code specific to this satellite and carried by a carrier wave; during the acquisition phase, the receiver being able to receive the navigation signal transmitted by the satellite, and to generate a local image of the data spreading code and a local image of the carrier wave; the method being implemented after the acquisition phase during a convergence phase and comprising the following steps: generating a first test signal comprising the local image of the carrier wave and a first test spreading code, and a second test signal comprising the local image of the carrier wave and a second test spreading code; the first and second test spreading codes being codes whose sum corresponds to the local image of the data spreading code; for each correlation interval of a plurality of correlation intervals, determining a first prompt correlator corresponding to the correlation value between the received signal and the first test signal, and a second prompt correlator corresponding to the correlation value between the received signal and the second test signal; determining first and second energy values corresponding to the energy of the first and second correlators, respectively, over all of the correlation intervals; determining a wrong synchronization indicator based on the difference between the first and second energy values; detecting a wrong synchronization when the wrong synchronization indicator is above a predetermined threshold.
 2. The method according to claim 1, wherein the wrong synchronization indicator (Ind) is determined using the following expression: ${{Ind} = \frac{{E_{2} - E_{1}}}{E_{2} + E_{1}}},$ where E₁ is the first energy value; and E₂ is the second energy value.
 3. The method according to claim 1, wherein the first energy value corresponds to the sum of the squares of norms of the first correlators over all of the correlation intervals, and the second energy value corresponds to the sum of the squares of norms of the second correlators over all of the correlation intervals.
 4. The method according to claim 1, wherein the first and second test spreading codes are decorrelated.
 5. The method according to claim 1, wherein the local image of the data spreading code and the first and second test spreading codes each define a code period formed by a plurality of chips, each chip assuming the value of a discretized state from among a set of discretized states, one of these states, called zero state, corresponding to the zero value of this code.
 6. The method according to claim 5, wherein: the period of the first test spreading code is made up of even chips of the local image of the data spreading code, each pair of said chips being spaced apart by a chip whose discretized state corresponds to the zero state; and the period of the second test spreading code is made up of odd chips of the local image of the data spreading code, each pair of said chips being spaced apart by a chip whose discretized state corresponds to the zero state.
 7. The method according to claim 5, wherein: the first half of the period of the first test spreading code is made up of corresponding chips of the local image of the data spreading code, and the second half is made up of chips whose discretized state corresponds to the zero state; the first half of the period of the second test spreading code is made up of chips whose discretized state corresponds to the zero state, and the second half is made up of corresponding chips of the local image of the data spreading code.
 8. A computer program product comprising software instructions which, when implemented by a piece of computer equipment, carry out the method according to claim
 1. 9. A receiver carrying out the method according to claim
 1. 